The present invention relates to a display element and a process for production thereof, said display element relying for its action on a material which separates out, dissolves, or changes in color upon electrochemical reduction or oxidation.
The recent widespread use of networks is replacing the conventional way of distributing printed matter by the transmission of so-called electronic documents. More books and magazines than before are being offered by so-called electronic publication.
Reading information in these forms usually needs a computer's cathode ray tube (CRT) or liquid-crystal display. Unfortunately, such displays of light-emitting type are ergonomically incomplete to permit continuous reading for many hours. Moreover, they can be used only in the vicinity of a computer.
Recently diffused notebook computers can be used as portable displays for reading; however, they are also unsuitable for reading for several hours or more because they still have the disadvantage inherent in those of light-emitting type and they consume a large amount of electric power.
A recent technical innovation in this field is the development of a liquid-crystal display of reflective type which operates with a less amount of electric power than before. However, it is poor in reflectivity—only 30% in the state of white-color display (with nothing appearing on the screen). Therefore, it is by far inferior in legibility to printed paper, which constantly fatigues readers.
One way to address the above-mentioned problems is by development of so-called paper-like display or electronic paper. It is based on the principle that a color is produced when chromatic particles are moved from one electrode to the other by electrophoresis or when dichroic particles are rotated in an electric field. The disadvantage of color development in this manner is that the space surrounding particles absorbs light, thereby reducing contrast. Another disadvantage is that a driving voltage higher than 100 V is necessary to achieve a practical writing speed (less than 1 second).
The low-contrast color development based on electrophoresis is overcome by the electrochromic display (ECD) which employs electrochemical reactions to produce colors. It is now in practical use as light-modulating glass and clock display, which do not need matrix driving. However, it is not suitable for displays, such as electronic paper, which need repeated on and off. Moreover, it is usually poor in black color quality, less flexible, and low in reflectivity.
The electrochromic display device, such as light-modulating glass and clock display, employs an organic material to form black segments. (In other words, electrochromism with an organic material is essential for good black color development.)
Unfortunately, special displays such as electronic paper are continuously exposed to sun light and room light by their nature, and organic materials are usually poor in light resistance and decrease in black density due to discoloration after use for a long time.
Also, as the liquid crystal unit, the one which uses the matrix drive unit as disclosed in, for example, Japanese Patent Publication No. Hei 4-73764 is known; however, the drive element used therein merely constitutes a part of the liquid crystal display unit.
Therefore, it has been required to find the mechanism suitable for the display medium such as electronic paper and paper-like display.
Unfortunately, in the display system that uses an electrochromic display unit, the one which satisfies the constitution suitable for the display medium such as the above-mentioned electronic paper and paper-like display does not exists.
With foregoing in mind, the present inventors paid their attention to a new display device in which color development takes place as metal ions (electrolyte) separate out and dissolve.
It was pioneered by the one proposed by Camlibel et al. which resorts to silver ions. (U.S. Pat. No. 4,240,717)
This precursory device, however, has never been put to practical use because of problems with reliability and repeatability.
In addition, it has problems with liquid leakage and thickness variation because it employs a solution of metal ions. Therefore, it presents difficulties in production and use as electronic paper or paper-lie display.
The present invention was completed in view of the foregoing. It is an object of the present invention to provide a high-quality display element which is easy to fabricate and to provide a process for production thereof.